A typical conventional Frequency Selective Surface (FSS) has a periodically replicated patterned metal film printed on the surface of a thin dielectric substrate material. A single instance of the replicated metal pattern is referred to as a unit cell. The unit cell may include one or more metal patches. The geometry of the metal patches is chosen to obtain a desired property of the FSS, such as electromagnetic scattering or absorption.
FSS applications include electromagnetic filtering devices for reflector antenna systems, radomes, absorbers, and artificial electromagnetic bandgap materials. The majority of FSS designs have been considered for microwave and millimeter wave applications, however the concept is scalable to higher frequency ranges such as infrared and even optical frequencies. FIGS. 20A-20C show three conventional devices, namely electromagnetic reflector 300, electromagnetic absorber 308, and antenna 310 respectively), each including a conventional FSS, the antenna also including radiative element 314. FIG. 20A shows absorber 300 comprising FSS 302, dielectric layer 304, and ground plane 306.
An electromagnetic absorber can be made by placing an FSS screen above a conventional metallic ground plane, separated by a relatively thin (compared to electromagnetic wavelength) dielectric layer. Such an FSS-based electromagnetic band gap (EBG) structure can act as an Artificial Magnetic Conductor (AMC) at a desired operating frequency, allowing thin absorbers (typical thicknesses can range from a tenth of a wavelength to as thin as a fiftieth of a wavelength or even less).
In a conventional absorber design, and in most FSS applications, the geometry and material parameters are engineered to produce a static frequency response. However, several groups have investigated the possibility of tuning or reconfiguring an FSS so that its frequency response can be shifted or altered altogether while in operation. This can be accomplished either by changing the electromagnetic properties of the FSS screen or substrate, by altering the geometry of the structure, or by introducing elements into the FSS screen that vary the current flow between metallic patches.
In a first class of Reconfigurable Frequency Selective Surface (RFSS), the frequency response of the FSS is changed by altering the electromagnetic properties of the substrate. Several groups have realized this by using a ferrite as the substrate material. By changing a DC bias applied across the ferrite substrate, the FSS can be tuned to higher or lower frequencies. However, there are some serious disadvantages associated with the concept of using ferromagnetic substrates. Ferrites have high mass, and large currents are required to maintain the DC bias across the substrate. Furthermore, setting up a DC bias over a large area of substrate is a complicated task. Nevertheless, a two-layer FSS with one or two ferrite substrates can be designed to switch between an absorber and a reflector at resonance by applying a DC bias to the substrate.
A related technique uses a liquid dielectric as the substrate. In this approach, a substrate cavity below the metallic screen is filled with a liquid dielectric or emptied to vary the permittivity. Varying the permittivity also varies the electrical wavelength inside the substrate, changing the frequency response. This technique has been demonstrated to tune the FSS frequency response, but it requires a complex design to properly handle the liquid substrate.
Another technique that alters the substrate properties uses a slotted FSS screen with a silicon substrate to produce a pass band at resonance under normal operation. However, when the silicon substrate is illuminated by an optical source with sufficient intensity, the silicon behaves like a conductor, making the pass band disappear. One final technique of interest involves using plasma to form a virtual FSS screen. Elements with a high plasma density behave like a metallic conductor. The plasma features can be altered thereby changing the frequency response of the virtual FSS.
The second category of RFSS design techniques are those in which the geometry of the metallic screen elements is altered in such a way as to effect a desired change in the frequency response. One technique that has been reported involves using two FSS screens with identical apertures or patch elements and a dielectric or spacing layer in between. The front and back screens are shifted vertically or horizontally with respect to each other, which produces a corresponding change in the frequency spectrum. The bandwidth and resonance positions both change when the screens are displaced.
A second reconfiguration technique has been introduced that is based on micro-electromechanical systems (MEMS) technology. The metallic elements of the FSS are designed to be able to lay flat on the substrate or tilt up to 90° from the substrate. Thus the incident radiation sees a variable-size element depending on the tilt angle of the metallic patches. This method for tuning the response of an FSS has been successfully demonstrated by Gianvittorio et al. (IEE Electronics Letters, Vol. 38, No. 25, December 2002). However, it requires complex fabrication techniques and the ability to produce an external electromagnetic field in order to mechanically control the element positions.
A further class of RFSS incorporates circuit elements into the metallic screen that can be used to vary the current between patch elements. A technique has been proposed for controlling the response of an FSS by interconnecting metallic patches in its screen with lumped variable reactive elements (C. Mias, IEE Electronics Letters, Vol. 39, No. 9, May 2003). Although variable reactive elements were not used in experiment, the effect of varying reactive loads between patches was shown through numerical simulations to shift the position of stop bands. This technique was taken a step further by including varactor diodes to tune the stop band of an FSS absorber.
Another option that has been investigated is to use PIN diodes as switches between metallic patch elements. PIN diodes either allow or inhibit current flow between patch elements depending on the voltage bias applied across the diode. Thus, they can be used to make a resonance disappear, or they can drastically change a resonance location based on the RFSS design. The active FSS described by Chang, et al. also incorporates a ferrite substrate so that the resonant frequency may be tuned by biasing the ferrite substrate or by switching the PIN diodes to go from a transmitting to a reflecting mode and back again (IEEE Proc. Microwaves, Antennas and Propagation, Vol. 143, No. 1, February 1996). One difficulty with using PIN diodes as switches in RFSS is the added complexity of incorporating bias lines into the design.
Several interesting applications have been suggested for RFSS that switch on or off using diodes. The design procedure for a horn antenna that has two tapered walls was described by Philips, et al. (IEE Electronics Letters, Vol. 31, No. 1, January 1995). The outer wall of the antenna is made of a solid metallic sheet while the second, narrower wall consists of a RFSS that incorporates diodes so that it can be switched from transmitting to reflecting. In the transmitting state, the horn antenna has a relatively wide aperture, but when the RFSS is switched to a reflecting state it acts as the inner wall of the horn antenna giving it a narrower aperture. The same type of active RFSS was proposed for building walls in order to control the transparency of the structure at a given radio frequency.